1. Field of the Invention
This invention relates to a tool for monitoring corrosion within a wellbore and, in particular, to a stand-alone downhole monitoring probe which records and stores corrosion data over an extended period for subsequent downloading and analysis upon retrieval of the tool.
2. Description of the Prior Art
During the drilling of an oil and gas or geothermal well, carbon dioxide and hydrogen sulfide gases react with water to form carbonic acid and hydro-sulfuric acid, respectively. Within the temperature and pressure condition of a well, these acids can create pH levels of approximately 4.0. The corrosive attack on the drill pipe or production tubing upon completion of the well can be very aggressive. Potentially corrosive environments in oilfield drilling and production operations are addressed by utilizing drill pipe and production tubing manufactured from corrosion resistant alloys or carbon steel materials in conjunction with corrosion inhibitor treatment programs. Since corrosion may occur at different rates from the wellbore perforations to the surface due to pressure, temperature, fluid density or velocity, a corrosive inhibitor which performs effectively at one depth may not be optimum for another depth. A means of accurately determining the corrosion at various depths allows corrosion inhibitors to be evaluated for effectiveness and optimum inhibitor flow rates.
One apparatus which has been used for corrosion monitoring is the "weight loss coupon". A coupon is a precisely machined piece of metal of the same or similar material as the pipe or lining. It is positioned within the flow of the fluid at a point where corrosion is to be monitored. The coupon remains there for a long enough period of time for some corrosion to occur (possibly a month or more) at which point the coupon is retrieved. The coupon is weighed and from the resulting weight loss the amount of cumulative corrosion over the exposure period can be determined. While this is an economical and accurate method of determining corrosion rates, it only gives average corrosion rates over the exposure period. Variations in the corrosion rate over the period cannot be determined. A method for determining the time related corrosion history can be an important tool to allow various corrosion inhibitors to be tried at varying flow rates to determine the optimum inhibitor and flow rate to minimize corrosion and costs.
Various alternative methods have been employed to monitor the metal loss over a predetermined period of time within the wellbore. Electrical resistance measuring elements have been employed but lack a reference element for accurate readings. Such resistance temperature devices are not used for discrete temperature measurements for compensation within the electrical measurement. Prior known systems have also attempted to utilize linear probe resistance technology with little success. Still other systems utilize electrochemical means of detecting when a corrosive fluid is present in a pipeline which normally contains a non-corrosive fluid. Since most wellbores contain corrosive environments it is the rate of corrosion which needs to be determined over a given period of time.